Machining a Lego Minifigure for Internal Wiring

The plastic figures on the Minifigure Multimeter need wires to connect to the components they are measuring, as well as to accept an LED for status display.
The first step is to disassemble the minifig into its eight detachable parts; nine parts if you count the head.

Lego minifig disassembled.

The hole in the neck for the LED is the easiest to make.
Measure the widest point on your T1 LED, to see if it is 3 millimeters or 3.1 millimeters.
Then, choose a 3 mm, #31, or 1/8-inch drill (listed from smallest to largest diameters).
If the LED still doesn’t fit, and you’re concerned that a larger drill will leave too thin of a neck wall, you can sand the outer edge of the LED.

Hole drilled in neck of minifig.

Drilling Holes in Lego Minifigure Hands

The hands of the Lego minifigure present a minor challenge to hold in a vise because the hands are tiny, slippery, flexible, and non-square.
The solution is to grip them in a chuck.
If you don’t have a lathe, then a 1/4-inch chuck attachment for a cordless drill
can be held in a vise on a drill press or mini-mill.
Thus, a centered chuck holding the wrist of the Lego hand will drill a centered hole straight through.

Hole in minifig hand in drill chuck. Notice the hole in the hand is also the center of the chuck.

Sadly, my initial attempts failed.
First, I used an expensive carbide bit that is suited for drilling printed circuit boards.
Narrow carbide is brittle and unforgiving of deep holes, and thus it broke off in the Lego wrist.
Second, the more flexible high-speed steel drill wandered during drilling, slashing the poor Lego piece.

Errors when drilling minifig hands.

The third (and subsequent) attempts were successful by starting the hole with a light kiss from a 1 mm stub carbide end mill.
Although this cutting tool is too short and too wide to drill the complete hole, it is relatively rigid and sharp.
Thus, it creates a nicely-centered starting point.

Milling a starter hole in a Lego hand.

Now the narrower, more flexible drill can go straight through, because it didn’t wandering off center at the beginning.
I used a no.66 high-speed steel drill from MicroMark (#15172).

Drilling hole in minifig hand.

Threading Wire through a Minifig Arm

I exclaimed several choice curse words when I discovered that the Lego arms are NOT hollow all of the way through.
I spent hours considering a variety of choices, none of which would have provided the perfection of a hollow arm.
Eventually, I attempted what every decent machinist knows does not have any right to be successful.

With the arm removed, I placed the drill such that the tip was centered vertically, horizontally, and depth-wise with where the wire should enter the shoulder.

Centering drill bit before attaching arm.

I locked the axes, set the drill stop, and raised the drill up.
The Lego arm was snapped on and raised straight up.
A light vise grip gently adds pressure to reduce the likelihood that the arm will rotate.

The narrowness of the drill and the obtuse angle of the elbow allow a drill to plunge straight through.
Believe it or not, this setup worked perfectly for all six arms (I made an extra minifig).

Difficult drilling of a wire hole for a minifig arm.

The legs and waist also need minor milling, drilling, or filing.
However, if you can machine the previous pieces, these parts are easy and require no instruction.

Finishing Touches

The top plate needs holes where the minifigures stand for the wires to pass through.

Drilling 1/8th holes in Lego plate for wires.

The bottom plate needs screw holes for attaching the circuit board.

Drilling holes in Lego base with a 4-40 drill to thread and attach circuit board.

Lastly, the side bricks need a slot for the USB cable.
This could be drilled, and the top cut off with a razor blade or hobby knife.
However, if you have a milling machine, the work is much cleaner and faster by simply milling into the brick.